The present invention relates to a display panel adapted for displaying a television picture for example, and also to a display device equipped with such a display panel.
It is observed recently that liquid crystal display (LCD) devices for example are attracting remarkable attention in place of cathode ray tubes (CRT), and rapid development is now in progress with regard to television receivers and the like employing such LCD devices.
There exists a background that such LCD devices developed and used originally as monitors for computers or the like are currently in wide diffusion. In constituting an LCD device to display television pictures, it is generally customary to apply an LCD panel produced for use in a computer. Such an LCD panel is so contrived as to conform with the control standards of computer graphics display termed VGA (video graphics array).
Referring now to FIG. 9A, an explanation will be given on an example of displaying a picture on an LCD panel which is based on the VGA control standards (hereinafter referred simply to as “VGA panel”) in accordance with video signal data obtained from video signals of one television system.
FIG. 9A shows an effective screen 102a of a VGA panel 102. Here, an effective screen is defined as an area where a picture can be actually displayed and viewed.
It is supposed here that the effective screen 102a of the VGA panel 102 has a resolution of 640×480 pixels. Although not shown in this diagram, the width-to-height ratio of each pixel is supposed to be 1:1. Therefore, the aspect ratio of the effective screen becomes 4:3 (=640:480).
In displaying a television picture on such an LCD device, it is necessary, for driving the VGA panel, to convert original analog television signals into digital video signal data of a predetermined format and to display the picture on the basis of such converted digital video signal data.
Further, due to the use of an LCD panel of a monitor produced for displaying the output of a computer, it is necessary to execute conversion of interlaced television signals, which are obtained by interlaced scanning of two fields forming a single picture, into progressive television signals obtained by scanning a single picture progressively from the top thereof. For this reason, the field-unit digital video signal data inputted field by field are converted into frame-unit picture data processed frame by frame.
Such frame picture data are outputted to the VGA panel at a predetermined timing, and the VGA panel is driven by an unshown driving circuit to display the picture.
It is prescribed here that the effective pixels of the frame picture data 101 representing one frame picture are composed of 720 pixels×483 pixels.
However, in the case of displaying the picture by supplying the frame picture data 101 to the VGA panel 102 as mentioned above, there arises a problem that the displayed picture becomes a vertically contracted image which is contracted in its vertical direction.
This problem is derived from the fact that, since the number of pixels of the frame picture data 101 supplied to the VGA panel 102 is 720×483, the ratio of the number of horizontal pixels to the number of vertical pixels of the post-conversion frame picture data 101 is 4:2.68 (=720:483).
More specifically, even if the original ratio of the horizontal picture data to the vertical one inputted as NTSC television signals, i.e., the aspect ratio thereof, is 4:3, such picture data are converted into the frame picture data 101 where the ratio of the number of horizontal pixels to the number of vertical ones is 4:2.68, so that the picture is contracted in the vertical direction. Consequently, the picture displayed on the effective screen 102a is also contracted in the vertical direction.
In addition to the above, there also exists the following problem in the case of displaying the picture, which is composed of such frame picture data 101, on the effective screen 102a of the VGA panel 102.
In the television signals, normally some unrequired signal components and the like are included in the video signals of the picture edge portions. Therefore, when displaying the television picture, the video signals are overscanned in such a manner that the image of the edge portion is positioned outside the screen instead of displaying the entire picture within the screen, whereby the image of the edge portion containing some noise components is not displayed.
Regarding the overscan executed in the case of FIG. 9A, first in the horizontal direction, the frame picture data 101 is composed of 720 pixels whereas the effective screen 102a is composed of 640 pixels, so that 80 pixels are surplus in the frame picture data 101. For this reason, an overscan area OS of 40 pixels is formed in each of the left and right edge portions equally.
Meanwhile in the vertical direction, the frame picture data 101 is composed of 483 pixels whereas the effective screen 102a is composed of 480 pixels, so that 3 pixels are surplus in the frame picture data 101. For this reason, an overscan area OS of 1.5 pixels for example is formed in each of the top and bottom edge portions equally.
However, viewing the rate of the overscan quantities in the horizontal and vertical directions, the horizontal overscan quantity is approximately 11% (80 pixels), whereas the vertical overscan quantity is merely 0.6% (3 pixels), whereby a remarkable difference is existent between the horizontal and vertical overscan quantities. If the overscan quantity is insufficient, it is impossible to conceal the noise of the picture edge portion completely. Meanwhile, if the overscan quantity is excessive, the displayed picture area is rendered narrow. As long as no noise appears in the picture data, it is preferred that the widest area be displayed effectively.
But if the horizontal and vertical overscan quantities are unbalanced, in the case of FIG. 9A for example, the rate of the overscan quantities needs to be raised as a whole by increasing the vertical overscan quantity to attain a proper value. In this case, the horizontal overscan quantity, which is sufficient from the beginning, is also rendered greater to consequently cause a disadvantage that the horizontal picture range displayed on the effective screen 102a is further narrowed. If a sufficient horizontal picture range is to be ensured in this case, the vertical overscan quantity needs to be maintained small, hence inducing a possibility of failure in completely concealing the noise or distortion that appears in the top and bottom edge portions of the effective screen 102a. 
Accordingly, it has been customary heretofore that, in the conventional LCD device or the like for displaying television picture, the frame picture data 101 is processed through reduction or interpolation to be thereby converted into desired frame picture data where the width-to-height ratio of the picture is proper.
FIG. 9B shows a structural example of a conversion block to execute such conversion of frame picture data. In this case, input video signals are converted into RGB data by a decoder 121 and then are supplied to a scan converter 122. Subsequently the scan converter 122 executes reduction or interpolation of the input RGB data. For example, the input RGB data are converted, through reduction of the vertical data thereof, into frame picture data of a proper width-to-height ratio and then are outputted to the VGA panel 102, whereby a desired picture having a proper aspect ratio of 4:3 is displayed on the VGA panel 102.
However, in the conventional LCD device mentioned above where a process of reduction or interpolation is executed in the scan converter 122, there is a disadvantage that the quality of the displayed picture is deteriorated, and the production cost is high since the scan converter 122 is expensive.
There is also known an LCD device of another structure, which will be described below, for balancing the vertical overscan quantity and the horizontal one.
FIG. 10A shows an exemplary case where frame picture data are displayed on an effective screen of a VGA panel in such an LCD device. In FIG. 10A, masking areas 104a and 104b are formed either electrically or mechanically in the top and bottom portions of an effective screen 102a. The horizontal and vertical overscan quantities can be rendered substantially equal to each other by adjusting the masking quantities of such masking areas 104a and 104b. 
For example, the vertical overscan quantity can be substantially equalized to the horizontal overscan quantity by masking the pixel data of approximately 53 pixels (483×0.11=53). In FIG. 10A, therefore, the masking areas 104a and 104b each corresponding to 25 pixels are formed in the top and bottom portions of the effective screen 102a respectively.
FIG. 10B shows a structural example of conversion blocks to convert video signal data for realizing the picture display shown in FIG. 10A. In this case also, the input video signals are converted into RGB data in a decoder 121 and then are inputted to an IP (interlace-progressive) converter 123.
The IP converter 123 converts the input RGB data, which are received field by field, into frame picture data so as to conform the same with the display performed by the LCD panel. The frame picture data thus obtained are supplied to a mask generator 124. Then the mask generator 124 executes a predetermined masking process for the top and bottom portions of the frame picture data received from the IP converter 123, and outputs the processed data to the VGA panel 102.
As a result, the picture shown in FIG. 10A is displayed on the VGA panel 102.
However, in the VGA panel 102 of FIG. 10A, the upper and lower portions of its effective screen 102a are masked, so that the screen size is rendered smaller correspondingly thereto.
Moreover, the production cost is raised due to the necessity of newly providing the mask generator 124 in the conversion block to execute conversion of the video signal data.
Further, the image of the frame picture data 101 is left contracted in its vertical direction to consequently fail in eliminating the known disadvantage that an elliptical picture 103, which is vertically flat, is displayed in place of an exactly circular picture on the effective screen 102a. 